Frictional heating in a unidirectional fibre-reinforced ceramic composite

Holmes and Shuler [1] recently found that significant internal heating occurs during the cyclic loading of fibre-reinforced ceramics. In their investigation, conducted with cross-ply carbon fibre/SiC matrix composites (hereafter referred to as Cf/SiC), it was observed that the extent of internal heating was strongly influenced by the peak fatigue stress and loading frequency. For example, during ambienttemperature fatigue between fixed stress limits of 250 and 10 MPa, the temperature rise measured at the specimen surface ranged from approximately 0.5 K at a sinusoidal loading frequency of 1 Hz to over 30 K at 85 Hz. For a fixed loading frequency of 75 Hz and minimum stress of 10 MPa, the temperature rise ranged from 0.8 K at a peak stress of 50 MPa to 28 K at a peak stress of 250 MPa. It was proposed that the temperature rise observed during cyclic loading was caused by the frictional sliding of fibres within the matrix [1]. Because of the significant mismatch in thermal expansion coefficients that exists between the fibres and matrix, C~/SiC composites contain extensive processing-related matrix cracking [2]. Fatigue loading of these initially microcracked specimens would promote interfacial debonding; once debonding occurs, internal heating would occur by the repeated frictional sliding of fibres along the fibre-matrix interface. For a composite that is initially free of matrix cracking, and assuming a mechanism of internal heating involving the frictional slip of fibres within debonded zones, internal heating should begin when a stress level that is sufficient to initiate matrix cracking is reached (at lower stress levels internal heating should be absent). To provide experimental evidence for this mechanism of internal heating, the present investigation utilized a simpler unidirectional fibre-reinforced ceramic that was initially free of matrix cracking. As discussed below, a simple experiment which involved monitoring the specimen temperature, while sequentially increasing the peak fatigue stress until matrix cracking initiated, was used to investigate the relationship between matrix cracking and internal heating. A 16-ply unidirectional Nicalon fibre/calcium aluminosilicate matrix composite (hereafter referred to as Nicalon/CAS-II) was chosen for this investigation (material processed by Corning Glass Works, Corning, New York, USA). The composite, formed by hot-pressing, had a nominal fibre content of 35 vol %. Edge-loaded tensile specimens (see Fig. 1) with a 33 mm gauge length were machined from the